mysterious girlfriend X
the story of two teenagers who share their love through drool

Don't forget the X! And I think that most people have heard of it now that it has a full season of anime under its belt... Before that, it was still fairly well known, but closer to fitting the category.

I really don't know any unheard of manga, since I usually don't read something until I have heard of it.

Yeah, but thats the closest thing that fits the bill, since i mostly read the popular stuff

Needless - more funny than the jokes on naruto/bleach/fairy-tale/one-piece and has more action and awesome art too
Freezing - great art and good drama with action
Medaka Box - seriously this one is a genius manga it has a lot of genre lol, and Nisio Isin the author of Monogatari series (Bakemonogatari/Nisemonogatari/etc) and Katanagatari and etc made this genius of a manga

I'm sure everyone has seen the Miyazaki film Nausicaä of the Valley of the Wind but many don't know it's adapted from an amazing manga (Well I sure didn't until years after seeing it). The movie only touches on the surface of the story in the manga. The manga is written and illustrated by Miyazaki as well, so you know it's top quality. It's seven volumes and is still in print. Check it out!

1/2 Prince, a story about a teenage girl and her brother that is really into gaming. They decide to play a virtual game together and they like to race to see who can get to a higher level than each other. Her brother also teases/complains how girls get it easy in games like how they can persuade a guy player to do their work. The sister being very annoyed somehow got 1 wish from the game master for entering the game 1st since it was just released. She decided her wish is to be a male character and her journey begins. Kinda long intro =P not sure if this is popular or not and sorry for any grammar mistakes, but this manga was really good and i think its still ongoing, but the chapter releases are really random and has no specific dates.